1. Field of the Invention
The present invention relates to a blanking aperture array for an electron beam exposure system and to a method of producing the blanking aperture array and, more particularly, to an improved method of producing electrodes in the blanking aperture array.
2. Description of the Related Art
The electron beam exposure systems include a point beam type in which the electron beam is used in the form of a spot, a variable rectangular beam type in which the electron beam is used in the form of a rectangle, the size of which is variable, and a stencil mask type in which a stencil having electron beam apertures formed therein in a desired pattern is used to shape the electron beam in a desired form.
The point beam type exposure system is suitable for forming a fine pattern, but since the throughput thereof is very low, this system is used only for research and development. The variable rectangular beam type exposure system has a throughput one or two degrees higher (i.e., one or two orders of magnitude greater) than that of the point beam type exposure system, but when patterns as fine as 0.1 .mu.m or at a high density are exposed to an electron beam, the throughput of this variable rectangular beam type exposure system is also low. When a 1-GB DRAM is exposed to the electron beam by this exposure system, the throughput becomes about three degrees less.
Methods by which all patterns can be exposed to the electron beam have been proposed; an exposure method using a two-dimensional blanking aperture array is one of these methods and is disclosed in Japanese Examined Utility Model Publication Sho 56-19402, the named applicants and assignee thereof being the common assignee herein.
The exposure system which implements the above-referenced method of electron beam exposure by the two-dimensional blanking aperture array is schematically shown in FIG. 1. In this electron beam exposure system, the electron beam emitted from an electron gun 51 is shaped into a one-shot beam (as shown by reference numeral 17 in FIG. 3), and thereafter, the beam is passed through lenses 53 and 55 and then projected upon a (BAA) blanking aperture array. The electron beam is deflected by deflectors 56 to select an area on the BAA 57 (such an area is identified by the reference numeral 17 in FIG. 3 in which the BAA is identified by numeral 18, corresponding to BAA 57 in FIG. 1) upon which the electron beam is to be projected. The electron beam, after passing through an aperture in the BAA 57, is passed through lenses 58 and 60, an aperture 61 and lenses 62 and 65, and is deflected by deflectors 63 and 64 and projected onto a selected position on a samples. A pair of blanking electrodes 59 are provided to deflect the electron beam so that it does not pass through the aperture 61.
Namely, in the two-dimensional blanking aperture array, multiple apertures for the passage of the electron beam are two-dimensionally formed in a semiconductor crystal substrate made of silicon or the like, a pair of blanking electrodes are disposed at respective opposite ends of each aperture, and pattern data is supplied defining whether or not a voltage is to be applied across the electrodes. The electron beam, after passing through the aperture is deflected or allowed to pass linearly by and independently, whereby it is determined whether or not the beam finally arrives at the sample. For example, when one of the blanking electrodes of each aperture is connected to the ground potential and a voltage is applied to the other electrode, the electron beam passing between the electrodes is deflected and is cut off by the aperture after passing through the lenses (at the reference numerals 58 and 60 in FIG. 1) located below the blanking aperture array, and thus the electron beam is not made incident on the sample surface. If no voltage is applied to the other electrode, the electron beam passing between the electrodes is not deflected, and thus the beam is not be cut off by the aperture after passing through the lenses provided below the blanking aperture array, and is projected onto the surface of samples.
Also the information carried by the pattern for each aperture (whether or not the electron beam passing through the aperture is to be deflected) is sequentially transmitted by shift registers, formed in the aperture array, selectively to the apertures. For this purpose, devices and wires must be accommodated to form the shift registers.
FIG. 2 is a schematic diagram of a two-dimensional blanking aperture array 18. As shown in the Figure, a plurality of apertures 15 is disposed two-dimensionally in the blanking aperture array, and a pair of electrodes 14A is formed at each of the apertures. Each lattice 13 is formed of wires and devices by which a voltage is applied to the electrodes of each aperture, independently, according to a given pattern data. The apertures are disposed at regularly spaced intervals of 10 .mu.m and each is 7 .mu.m by 7 .mu.m in size. Also, each electrode is about 1 .mu.m thick, 7 .mu.m wide and 20 .mu.m long (in the depth direction, transverse to the plane of FIG. 3).
FIG. 3 is a plan view showing an exemplary blanking aperture array 18. In this case, the blanking aperture array 18 consists of four shot areas 17 each having sixteen (4.times.4) apertures 15, and is controlled according to a given pattern data to determine whether or not the electron beam passing through each aperture in one shot area 17 is to be deflected (i.e., whether or not a voltage is to be applied between the electrodes of each aperture) when the beam passes through the aperture. At the other three shot areas, the beam passage is controlled according to different pattern data for the passage of the electron beam. The electron beam is deflected to each shot area by the deflector 56 in FIG. 1.
The above-mentioned two-dimensional blanking aperture array can be formed by the processes shown in FIGS. 4(1) to 4(4) and 5(1) to 5(4). As shown in FIG. 4(1), an impurity diffusion layer 12 is formed on a semiconductor substrate 10 by doping impurities, and an epitaxial growth layer 14 is grown on the impurity diffusion layer 12 as shown in FIG. 4(2). Thereafter, other devices are formed in the epitaxial growth layer 14, i.e., MOS transistors, etc., which implement an inverter and a gate.
The aperture AP with spaced sidewalls and spaced electrodes E.sub.1 and E.sub.2 formed on the respective aperture sidewalls, as shown in FIG. 4(4) is formed as shown in FIGS. 5(1) to 5(4).
As shown in FIG. 5(1), a narrow trench 16 is formed by trench etching at opposite sides of each aperture to be formed, extending through the epitaxial growth layer until the trench reaches the substrate 10. Next, an insulation film 18 is formed on the entire surface as shown in FIG. 5(2), and then an electrode material 20 is deposited in each trench 16 as shown in FIG. 5(3).
Next, the portions of the epitaxial growth layer 14 and the impurity diffusion layer 12 between the thus formed electrodes E.sub.1 and E.sub.2 are removed by etching to form the aperture AR therethrough as shown in FIG. 5(4).
Furthermore, the semiconductor substrate 10 is taper-etched between the electrodes E.sub.1 and E.sub.2, from the rear side thereof as shown in FIG. 4(4), whereby the completed aperture AP is formed.
The output of each of the shift register stages is connected to one of the electrodes E.sub.1 and E.sub.2 of each aperture, and the low voltage side GND of the power source or, alternatively, the high voltage side V.sub.DD, is connected to the other electrode. This wiring process is effected at the same time as, or separately from, the wiring to each device of the shift register and the clock signal wiring.
FIG. 6 shows the arrangement of such a blanking aperture array circuit. As seen in the Figure, AP indicates a beam aperture, and a pair of electrodes E.sub.1 and E.sub.2 are formed in spaced relationship at respective, appropriate edges of each aperture. E.sub.1 is an electrode to which a constant voltage is always applied, and E.sub.2 is an electrode to which a voltage which is varied in accordance with the given pattern data is applied. When the electron beam is turned on, a same voltage (0V, for example) is applied to the electrodes E.sub.1 and E.sub.2 so that the beam passing through the aperture 61 in FIG. 1 is not deflected when passing between the electrodes, and arrives at the samples in FIG. 1. On the other hand, when the electron beam is turned off, a voltage (+5V, for example) applied to the electrode E.sub.2 is different from the voltage applied to the electrode E.sub.1, and thus the beam when again turned on is deflected when passing through the electrodes and is given a trajectory such that it cannot pass through the aperture 61 in FIG. 1. Thus, in the latter case, the electron beam will not arrive at the samples. The beam on/off data (0 or 5V) is transferred by the shift register, formed in the BAA, to the electrodes of each aperture. Namely, the data supplied from the left by data transfer wires SR.sub.1 to SR.sub.3 is shifted until it is transferred fully to the right by clock signal wires CLK.sub.1 and CLK.sub.2. During the data transfer, another area (at the reference numeral 17 in FIG. 3) is exposed to the deflected electron beam or a variable rectangular aperture is used. Note that "U" in FIG. 6 indicates an area where a device (as in FIG. 4(4) is formed.
As seen from the foregoing, the blanking aperture array is conventionally formed by etching a semiconductor substrate for forming electrodes, implanting electrode materials in the substrate and, thereafter, forming apertures at which the electrode surfaces are exposed.
Namely, the conventional method of forming electrodes necessitates the etching of the substrate, and the portions where the electrodes are to be formed must be etched to a greated depth than the etched area, for example, 7.0 .mu.m (longitudinally).times.1.0 .mu.m (laterally), and 20 to 30 .mu.m (in depth). Since the electrode side face is formed along the etched trench wall of the substrate (in effect, an insulation layer side face is defined by the trench and then an electrode material is implanted in the trench to form the electrode side face), the etching must be linear because the electrode side face is defined by the form of the etched aperture. Also, the insulation layer must be formed to include the etched trench and the electrode material implanted in the trench, and unless the electrode material is fully buried in the trench, it cannot operate for the full length of the electrode's intended life. Accordingly, the electrode materials must be fully buried for each of the two electrodes formed in each of the apertures of the array (e.g., 200 apertures.times.200 apertures.times., and this process is difficult.
It is also very difficult to form many apertures in a limited space, as each aperture must have a substantial size because of the relation with the time of exposure to the electron beam.
Furthermore, elaborate devices for the shift registers must be formed in the substrate, to prevent a malfunction of the devices due to exposure to the electron beam.